In recent years, with growth of IoT (Internet of Things), development of a wearable device is advancing. Typical examples thereof are a clock and an eye glass that can be connected to an internet. In a medical field and a sport field, too, a wearable device that can always monitor a body's condition is wanted; and thus, this is a growing field from now on.
In a medical field, for example, as in the case of electrocardiogram measurement with which the heart movement is detected by an electric signal, a wearable device that can monitor the condition of a body's organ by sensing a weak electric current is being studied. In electrocardiogram measurement, the measurement is carried out by attaching to a body an electrode applied with a conductive paste, but this is for only one measurement with a short measurement time. On the other hand, the wearable device for medical treatment as mentioned above aims development of a device with which a health condition can be always monitored for several weeks continuously. Accordingly, the biological electrode to be used in the wearable device for medical treatment is required not to change in conductivity even for a long period of usage as well as not to cause an allergy to a skin. In addition to these, the biological electrode is also required to be light in the weight thereof and producible at a low cost.
With regard to the wearable device for medical treatment, there are a body-attachment type and a cloth-incorporation type. With regard to the body-attachment type, a biological electrode using a water-soluble gel, which is the above-mentioned conductive paste material including water and an electrolyte, is proposed (Patent Literature 1). The water-soluble gel includes, as the electrolyte, sodium, potassium, and calcium in a water-soluble polymer to hold water therein, whereby converting a change of the ion concentration from a skin to electricity. On the other hand, with regard to the cloth-incorporated type, a method to use an electrode which has a silver paste or a conductive polymer such as PEDOT-PSS (poly-3,4-ethylenedioxythiophene-polystyrenesulfonate) incorporated into a cloth is proposed (Patent Literature 2).
However, in the case that the water-soluble gel including water and the electrolyte is used, there has been a problem that the conductivity is lost when water is lost by drying. On the other hand, in the case that a metal having a high ionization tendency such as copper is used, there is a problem of a risk to cause a skin allergy depending on a person; and also in the case that the conductive polymer such as PEDOT-PSS is used, there has been a problem of a risk to cause a skin allergy because acidity of the conductive polymer is high.
Also, use of a metal nanowire, a carbon black, a carbon nanotube, and or like as the electrode material is being studied because these materials have an excellent conductivity (Patent Literatures 3, 4, and 5). The metal nanowire has a high probability of contact among the wires themselves so that the energization can take place with a small addition amount thereof. However, because the metal nanowire is a material having a sharp and fine edge, it can cause a skin allergy. For the same reason, the carbon nanotube can irritate a living body. The carbon black is not as harmful as the carbon nanotube, but it still has a little bit of skin irritation. Therefore, even if the material itself does not induce an allergy reaction, biocompatibility can be deteriorated because of the shape and irritation property of the material; and thus, it has been difficult to satisfy both the conductivity and the biocompatibility at the same time.
It may be presumed that a metal film can function as an excellent biological electrode because it has a very high conductivity, but it is not necessarily the case. It is not a very weak electric current that is discharged from a heart beating; but those that are discharged are a sodium ion, a potassium ion, and a calcium ion. Therefore, it is necessary to convert the change in concentration of these ions to an electric current; but noble metals are poor in efficiency to convert these ions from a skin to an electric current because they are sluggish in ionization. Therefore, the biological electrodes using noble metals have high impedance so that energization with a skin is highly resisted.
On the other hand, a battery added with an ionic liquid are being studied (Patent Literature 6). The ionic liquid has characteristics of high thermal and chemical stabilities as well as excellent conductivity, and thus, the application thereof is extending to the use in battery. However, the ionic liquids such as those described in Patent Literature 6 have small molecular weights so that they are soluble in water; and thus, when the biological electrode added with the ionic liquid is used, this is extracted from a skin by a sweat. Therefore, this causes not only deterioration of conductivity but also a rough skin due to penetration of the ionic liquid into a skin.
The biological electrode cannot receive information from a body when it is departed from a skin. In addition, even a change in a contact area can cause a change in an electric amount of energization thereby causing the change in a base line of the electrocardiogram (electric signal). Therefore, in order to obtain a stable electric signal from a body, it is necessary that the biological electrode be always contacted to a skin and that the contact area thereof does not change. For this reason, it is preferable that the biological electrode has adhesiveness. In addition, the biological electrode needs to have elasticity and flexibility so as to follow the skin's expansion and contraction as well as the change in bending thereof.